A) Field of the Invention
The present invention relates to a liquid crystal display (LCD)
B) Description of the Related Art
Many of liquid crystal display apparatus capable of segment display or both segment display and dot-matrix display under multiplex driving now mount a normally black type liquid crystal display having a very low display luminance in a background display area and dark display areas. Most of them are monochromatic displays using a single color LED as a backlight, and adopt a twist nematic (TN) type as a liquid crystal display structure.
A vertically aligned liquid crystal display is known which disposes a liquid crystal cell of a vertical alignment mode in which liquid crystal molecules in a liquid crystal layer are aligned vertically or approximately vertically between upper and lower glass substrates sandwiching the liquid crystal layer, between approximately crossed-Nichol disposed polarizers. As this vertical alignment type liquid crystal display is observed normal direction of a glass substrate, the optical characteristics become almost equal to those of the crossed-Nichol disposed polarizers. Therefore, an optical transmissivity becomes very low, and it is possible to realize a high contrast ratio relatively easily. The vertical alignment type liquid crystal display allows good normally black image display independent from an emission wavelength of a backlight.
An invention of a liquid crystal display is disclosed (for example, refer to Japanese Patent No. 2047880) in which an optical film having negative uniaxial optical anisotropy or an optical film (negative biaxial film) having negative biaxial optical anisotropy is inserted at one or both sides between the upper polarizer and upper glass substrate and between the lower polarizer and lower glass substrate. Even if the liquid crystal display is observed along an oblique direction, it is possible to suppress a rise in optical transmissivity and a fall in contrast ratio so that good image display can be realized.
For this viewing angle compensation method described in Japanese Patent No. 2047880, effective conditions have been proposed (for example, refer to Japanese Patent No. 3330574) for an inplane retardation and an arrangement of an in-plane slow axis, respectively of a negative biaxial film.
As an optical film (negative C plate) having negative uniaxial optical anisotropy, a biaxially stretching film is distributed in markets which is formed by stretched triacethyl cellulose (TAC) resin formed in a film shape by a melting cast method or norbornene base cyclic olefin polymer (norbornene COP) resin formed in a film shape by a melting extrude method, the resin having been used as a material of a protective film of a polarizer, along a film extrude direction and its perpendicular direction.
As a negative biaxial film, there are a film formed by uniaxially stretched specific TAC resin along a direction perpendicular to a film longitudinal direction, and a film formed by biaxially stretched norbornene COP resin. These films are used, for example, in a liquid crystal television, and distributed in markets by a larger amount than negative C plates.
Most of negative C plates made of a commercially available TAC film have a retardation Rth of 50 nm or smaller in a thickness direction. A negative C plate made of a commercially available norbornene COP film has also a retardation Rth of 300 nm or smaller in a thickness direction. An optical film having Rth larger than 300 nm is not presently distributed in markets, although it is known that this film can be made of cholesteric liquid crystal polymer.
Commercially available negative biaxial films have an in-plane retardation Re of 0 nm<Re≦300 nm, both for stretched TAC and norbornene COP, and most of them have Re of 20 nm<Re≦300 nm. A maximum retardation in a thickness direction is about 220 nm to 350 nm.
Supplementary description will be made on an optical film prior to making detailed description of conventional examples and embodiments.
An optical film having negative uniaxial optical anisotropy (negative C plate) has a relation of nx≈ny>nz, and an optical film having negative biaxial optical anisotropy has a relation of nx>ny>nz, where nx and ny are in-plane refractive indices of an optical film, and nz is an out of plane refractive index along a thickness direction.
As an Nz factor is defined by Nz=(nx−nz)/(nx−ny), negative uniaxial optical anisotropy is N≈∞ and negative biaxial optical anisotropy is 1<Nz<∞.
Nz=1 indicates positive uniaxial optical anisotropy, and an optical film having positive uniaxial optical anisotropy is called an A plate. An A plate is nx>ny=nz. Nz<1 indicates mainly positive biaxial optical anisotropy. However, a case of Nz=0 and Nz=−∞ is excluded because only one optical axis exists. An optical film of Nz=0 is called a negative A plate, whereas an optical film of Nz=−∞ is called a positive C plate.
An in-plane retardation Re of an optical film is defined by Re=(nx−ny)*d where d is a film thickness, and a thickness direction retardation Rth is defined by Rth=[{(nx−ny)/2}−nz]*d.
FIGS. 10A and 10B are schematic configurations of liquid crystal displays according to conventional examples. Japanese Patent No. 3330574 discloses techniques assuming the structures of these liquid crystal displays.
FIG. 10A is a schematic diagram illustrating the liquid crystal display of a first conventional example
Between an upper polarizer 10 and a lower polarizer 20 crossed-Nichol disposed, a mono domain vertical alignment liquid crystal cell 30 is disposed. The mono domain vertical alignment liquid crystal cell 30 is constituted of an upper glass substrate (transparent substrate) 4, a lower glass substrate (transparent substrate) 6 and a mono domain vertical alignment liquid crystal layer 5 squeezed between both substrates 4 and 6. A single first optical film 3, e.g., norbornene COP biaxially stretched film, is disposed between the upper glass substrate 4 of the vertical alignment liquid crystal cell 30 and the upper polarizer 10.
The upper and lower polarizers 10 and 20 each have the structure that a polarizing layer 1 is disposed on a TAC base film 2. For example, the polarizing layer 1 is made of stretched polyvinyl alcohol.
In the directional coordinate system shown wherein a right/left direction of the liquid crystal display is defined as a 180°-0° (9 o'clock-3 o'clock) direction, an absorption axis Fab of the polarizing layer 1 of the upper polarizer 10 is set along the 135° direction, and an absorption axis Rab of the polarizing layer 1 of the lower polarizer 20 is set along the 45° direction. An in-plane slow axis SA1 of the first optical film 3 is set along the 45° direction.
As described earlier, the thickness direction retardation Rth of the first optical film can be set to only 300 nm or smaller. Therefore, if a retardation Δnd of the liquid crystal layer 5 of the liquid crystal display of the first conventional example is large, good viewing angle compensation cannot be obtained in a background area and dark display areas.
FIG. 10B is a schematic diagram illustrating the liquid crystal display of a second conventional example. A different point from the first conventional example resides in that between a lower glass substrate 6 of a vertical alignment liquid crystal cell 30 and a lower polarizer 20, a single second optical film 7, e.g., norbornene COP biaxially stretched film, is disposed. An in-plane slow axis SA2 of the second optical film 7 is set along the 135° direction. The first conventional example is a liquid crystal display of one-side compensation, whereas the second conventional example is a liquid crystal display of both-side compensation.
In the liquid crystal display of the second conventional example, the retardation in the thickness direction of the optical films can be set to about 600 nm at a maximum as a total sum of the first and second optical films. However, as the liquid crystal display is observed at a large polar angle along the normal direction of the right/left direction (180°-0° direction) set along 45° direction relative to the absorption axes Fab and Rab of the upper and lower polarizers 10 and 20, an optical transmissivity in the bright display area becomes extremely low, and display image cannot be visually recognized at all in some cases.
The present inventor obtained right/left direction viewing angle characteristics during bright image display of the liquid crystal displays of the first and second conventional examples. “LCDMASTER6.16” manufactured by SHINTECH, Inc. was used as a simulator. The same simulator was used for other simulations in this specification.
For both the first and second conventional examples, the vertical alignment liquid crystal cell 30 was structured to have anti-paralleled mono domain alignment having a pretilt angle of 89.9° on the substrate surface and the six o'clock direction (270° direction) alignment of central molecules of the liquid crystal layer 5, by using liquid crystal material having negative dielectric anisotropy Δ∈. A retardation Δnd of the liquid crystal layer was set to about 430 nm for the first conventional example, and to about 445 nm for the second conventional example. SHC13U manufactured by Polatechno Co. Ltd was used as the upper and lower polarizers 10 and 20, and an in-plane retardation of the TAC base film 2 was set to 3 nm, and a thickness direction retardation was set to 50 nm.
The first and second optical films 3 and 7 are made of a biaxially stretched norbornene COP film, as described above. For simulation, an in-plain retardation of the first optical film 3 of the first conventional example was set to 50 nm and a thickness direction retardation was set to 300 nm. For the second conventional example, in-plane retardations of the first and second optical films 3 and 7 were set both to 30 nm, and thickness direction retardations were set both to 150 nm. Both the first and second conventional examples provide the optimum optical film conditions of an optical transmissivity smaller than 0.03% as viewed at a right direction polar angle of 50° under the absence of applied voltage.
FIG. 11 is a graph illustrating the right/left direction viewing angle characteristics with an optical transmissivity of observation from normal direction of liquid crystal display being set to about 15%.
The abscissa of the graph represents a right/left direction observation angle in the unit of “°”, and the ordinate represents an optical transmissivity in the unit of “%”. A curve a indicates right/left observation viewing angle characteristics of the liquid crystal display (one-side compensation) of the first conventional example, and a curve b indicates right/left observation viewing angle characteristics of the liquid crystal display (both-side compensation) of the second conventional example.
It can be seen from the graph that an optical transmissivity of the liquid crystal display of the first conventional example is higher than that of the second conventional example, at an observation angle larger than a polar angle of 50°. Right/left symmetry of the liquid crystal display (both-side compensation) of the second conventional example is better than that of the first conventional example.
The present inventor actually manufactured the liquid crystal displays of the first and second conventional examples. It has been found that the second conventional example has the tendency that color shift is observed remarkably at a large angle, and this tendency is not proper from the viewpoint of outer appearance of the display.
In order to obtain a larger display capacity, i.e., a duty ratio, a liquid crystal display by using multiplex driving, it is necessary that steepness of the electro-optical characteristics is good. In the case of a vertical alignment type liquid crystal display, steepness is reflected upon a display luminance because a transmissivity in the bright display area is greatly influenced.
If a display capacity is desired to be made larger than that of a ¼ duty driving condition, it is preferable to set a retardation Δnd of a liquid crystal layer larger than 300 nm, and more preferably larger than 360 nm. If a display capacity is desired to be made larger than that of a 1/16 duty driving condition, it is preferable to set a retardation Δnd of a liquid crystal layer larger than 550 nm, and more preferably larger than 600 nm. It is considered that even if the liquid crystal displays of the first and second conventional examples are manufactured by using commercially available optical films, it is difficult to obtain good image display performance.